[ARM][AArch64] Implement __cls, __clsl and __clsll intrinsics from ACLE

Summary:
Writing support for three ACLE functions:
  unsigned int __cls(uint32_t x)
  unsigned int __clsl(unsigned long x)
  unsigned int __clsll(uint64_t x)

CLS stands for "Count number of leading sign bits".

In AArch64, these two intrinsics can be translated into the 'cls'
instruction directly. In AArch32, on the other hand, this functionality
is achieved by implementing it in terms of clz (count number of leading
zeros).

Reviewers: compnerd

Reviewed By: compnerd

Subscribers: kristof.beyls, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D69250